1. Field of the Invention
The invention concerns a process for the electrostatic printing of functional materials, for example conductors or semiconductors, configured as liquid toners for various manufacturing applications.
2. Description of the Related Art
A. Solder Bumping
Present industry road maps for silicon chips specify 4,000 I/O connections on a chip as small as 15 mm by 15 mm. Since the traditional wire bonding techniques cannot handle this density, wafer bumping or “flip chip” technology is necessarily applied. Each I/O pad requires a relatively tall solder bump, which will make contact eventually with a printed wiring structure. There are three techniques currently used to build these bumps:
1. Stencil printing of solder filled paste. For relatively large features (above 150 micron pad diameter) this method is preferred, since it is simple and requires no additional processing.
2. Flip Chip Technologies Inc. of Phoenix, Ariz. practices a variation of stencil printing, in which a stencil mask is spin coated on the wafer, then photo-lithographically patterned. The electrode cavities are filled with solder ink using a doctor blade. The solder ink is re-flowed into near perfect spheres, and then the mask is stripped chemically.
3. A similar mask is employed as in a doctor blade approach. Problems of this method include a 40 minute plating process to form the bumps, and the difficulty and/or inability to effect a plating of lead free alloys, e.g., Sn/Ag/Cu.
B. Building Glass Ribs and Other Microstructures
Plasma display panels are a growing flat panel display technology whose market acceptance has been limited by high manufacturing costs. Two important, high cost elements of this technology include:                A. Glass barrier ribs, which isolate discharge chambers from one another; and        B. Electrical lines, usually made of a silver/glass mixture, that is sintered in the 500+° C. range.        
The glass barrier ribs were screen printed, but with as many as seven print steps to achieve the 100 microns of needed rib height. Recently, sand blasting techniques have been used, but the costs are still high. The conductive lines are typically photo-defined, with the unexposed materials etched away, causing a large waste of expensive silver which should then be re-cycled.
Field emission displays are a relatively new technology. They consist of an array of field emission points in a vacuum, spraying electrons onto a phosphor screen. With three color dots on the screen and addressability of the emitting points, one has a full color display.
Active matrix liquid crystal displays have been intensively developed for production. The manufacturing process includes: a. photolithography or the patterning of photo sensitive resists and the “washing” and etching processes that are attendant to them, b. printing relatively large area features (30μ or more) c. the low pressure sputtering processes for coating glasses with metals like aluminum or indium/tin oxide (ITO), a transparent electrode or dielectrics like SiO2. In all cases the process has many steps, many in which the substrate glass has to be heated and then cooled back to room temperature before the next step.
See, U.S. Pat. Nos. 6,876,370, 6,781,612, 6,743,319, 6,524,758, 6,379,745, 6,274,412, 6,153,348, 6,143,356, and 5,882,722, each of which is expressly incorporated herein by reference.
Electrostatic printing has been used for color proofing in Du Ponts EMP process during the late 1980's. Du Pont used the electrostatic printing which is described by Reisenfield in U.S. Pat. No. 4,732,831. It used liquid toners that were transferred directly to a smooth, coated sheet of paper.
The transfer of liquid toner, which is important to this invention, was disclosed by Bujese in U.S. Pat. No. 4,879,184 and U.S. Pat. No. 4,786,576. These documents teach the transfer of liquid toners across a finite mechanical gap, typically 50μ to 150μ. This technology has been applied where toner, with etch resist properties, was transferred to copper clad glass epoxy boards.
Other prior work related to the printing plate and “gap transfer” includes M. B. Culhane (Defensive Publication# T869004, Dec. 16, 1969) and Ingersol and Beckmore to the electrostatic printing plate (See, U.S. Pat. Nos. 3,286,025, RE 29,357; and RE 29,537, respectively).